Rainwater collection is certainly nothing new; humans have been
doing it for thousands of years. However, with the advent
of cheap, potable water delivered right to your doorstep, those
who harvest rain have become somewhat of an anomaly. This
is changing in Washington State, largely for three reasons:

On October 12, 2009, Ecology issued an Interpretive Policy
Statement clarifying that a water right is not required for
rooftop rainwater harvesting. There is also a Focus Sheet
on this subject – see the links in the right column.

Once you have collected the rainwater there are no
limitations on its use. If and when the department
determines that rooftop or guzzler rainwater harvesting systems
are likely to negatively affect instream values or existing
water rights, local restrictions may be set in place to govern
subsequent new systems (there are currently no restrictions).
However, Ecology generally does not expect the collection of
harvested rainwater to cause problems or reduce the amount of
runoff that would have occurred from the site in its natural,
pre-development state.

The rainwater storage system may also be used for rainwater
collection to store groundwater withdrawn under the groundwater
permit exemption for future beneficial use (i.e. an owner may
pump groundwater in the winter under the permit exemption, store
it, and then use it in the spring and summer). According
to the new rainwater interpretive policy, an “on-site”
rooftop/guzzler system means the storage and use of the
rainwater occurs on the same parcel as the roof from which the
water was captured.

Two states have produced detailed guidance on rainwater
harvesting: Texas in 2005 and Georgia in 2009 – see the links in
the right column.

Rainwater Harvesting Calculator

Ecology created a
Rainwater Harvesting Calculator (RWH) to help Washingtonians
size their RWH system based on the typical climate where they
live. The model allows the user to test various RWH
scenarios (different roof size, cistern size, indoor use,
outdoor use) and see how well the desired use is achieved during
a normal precipitation year.

To use this calculator start on the Data Entry page.
Toggle to the city nearest you and begin entering data in the
green highlighted cells. Every green highlighted cell must be
filled out. When done, you can review the results in the
blue highlighted cells on the bottom half of the page.
Furthermore, you can scroll to the different charts to see a
more visual representation of how well your RWH system is
meeting your needs. You can change the data entries as
many times as you wish.